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Old 10-18-2010, 09:22 PM   #1
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Help with detach garage main feed

Hello, Not long ago I posted about trying to figure out weather my main feed from house to garage went bad underground. After help from a lot of you I figured out that the line went bad. I am deciding to try and fix this myself. The line was direct buried so with the frost coming I called out the diggers hotline and had my yard flagged. I hand dug a trench shovel with 24 inches deep. NEC minimum is 18 inch. I just needed to get the ground dug before frost hits. I'm going to try and run a 2'' conduit this weekend. I'm really questioning if the garage was to code in the first place. Ill start with how it was wired. I have in my house a 30 amp double pole breaker coming off my 100 amp main panel. wire is a 12/3 u wire. Has a black,white,red,ground wire. This wire goes directly from the 30 amp breaker to the sub panel in the garage. I'm not sure how may amps the this panel is rated for but it has 8 breaker slots and a bus bar. In the panel all the white neutral and green ground go to the same bus. I also have a 8 ft ground rod.

I've done a lot of reading but I'm still confused with what needs to be done. I know 12 awg is to small and 30 amp breaker is only allowed to circuits minimum. Questions. At the 100 amp main to I need to come off the breaker and have a disconnect panel then the line to the garage? What size breaker/wire/sub panel with slots should I get? My garage sits about 30 feet from my house so wire run underground would be 40 ft max. I'm basicly a weekend warrior. No high demanding tools. Garage door opener, miter saw, 10 inch table saw. Security light and interior lights. I will be getting a permit so I have the second eyes to look over my work. I really appreciate the help. I try to do as much as I can without paying out to subs.
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Old 10-18-2010, 10:31 PM   #2
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You're on the right track here.

If you just want to keep the same capacity you had, the 30A breaker currently in your main panel is fine, and you can run #10 wires out to the garage. You'll need four wires, 2 hots, a white neutral, and a green ground. No additional disconnect is needed between your main panel and the wires leaving the house.

The existing subpanel in the garage is probably fine, as long as it has a main breaker, and that breaker is rated at least 30A. Otherwise you will need to change to a subpanel that has a main breaker, or add a disconnect between the incoming power and subpanel.

Regardless of which way you go with that, you also need to separate all the neutrals and grounds in the sub-panel to their own respective bars, and make sure that the neutral bar is not bonded to the metal panel box or to the ground bar in any way.

You will also need to add an additional 8' ground rod at the garage, I believe.

Now, what I would do, if you're running a 2" conduit anyway, is pull #6 wire instead. #6 for the hots and neutral, #10 for the ground. In this case, you can swap the current 30A breaker in your main panel for a 60A, and have twice the available capacity out there. You could replace the existing garage sub-panel with a 100A Square D main breaker panel, available for around $50 most of the time. Again, separate neutral and ground bars, etc.

Remember also that to do things legal, you need all the outlets in your garage to be GFCI protected, either by GFCI outlets or GFCI breakers. Depending on your area, the outlets may also need to be the tamper-resistant type, ridiculous as it may be.
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Old 10-18-2010, 10:44 PM   #3
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For the usage that you describe you could run a 3/4" PVC conduit and have room enough to run a 50 amp subpanel in the outbuilding. This is more than enough power from your description.

Here is a good place to start to figure out how to do this.
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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